Śatapatha Brāhmaṇa ("of a hundred paths") 6.7.2.8

6.7.2.8

taṃ haike | etayā vikṛtyābhimantryānyāṃ citiṃ cinvanti droṇacitaṃ vā rathacakracitaṃ vā kaṅkacitaṃ vā praugacitaṃ vobhayataḥ praugaṃ vā samuhyapurīṣaṃ vā na tathā kuryādyathā pakṣapucavantaṃ garbham parivṛścettādṛktattasmādenaṃ suparṇacitameva cinuyāt

Now some, after addressing him by that fashioning (formula), build a different altar (than of an eagle's shape), either one constructed in the form of a trough, or like a chariot-wheel, or like a kite, or like the front part of a thill, or like a thill on both sides, or one consisting of a heap of loose soil. Let him not do so, (but) in such wise as one might carve a young one with wings and tail: let him therefore build it (the fire-altar) in the form of an eagle.